VoIP phone is loosing connectivity

I have an interesting problem. I have set up RasPBX (FreePBX on RaspberryPi) with several SIP softphones, which are mostly connected via VPN. However, I have one VoIP desk phone - Aastra 6730i. This physical phone is also connected to RasPBX through VPN, but because the phone does not support VPN, it is connected via another RaspberryPi which serves as VPN bridge. Technically that means that this second RPi is connected to VPN, and VoIP phone is connected to this RPi through USB-to-Ethernet dongle.

Everything seems to be working just fine, however, I noticed a problem… this Aastra VoIP phone is loosing connectivity to RasPBX very often. In FreePBX logs I can see:

9003	[2022-08-06 16:03:43] VERBOSE[26106] res_pjsip/pjsip_configuration.c: Endpoint 7000 is now Unreachable	
9005	[2022-08-06 16:06:40] VERBOSE[26106] res_pjsip/pjsip_configuration.c: Endpoint 7000 is now Reachable

Now, RPi which acts as VPN bridge does not loose connectivity, so there could be another problem. Therefore I connected to this “VPN bridge” RPi and try to ping VoIP phone. Here are the results:

ping 192.168.100.51
PING 192.168.100.51 (192.168.100.51) 56(84) bytes of data.
^C
--- 192.168.100.51 ping statistics ---
41 packets transmitted, 0 received, 100% packet loss, time 673ms

So, it seems that the problem is the connection between “VPN bridge” and Aastra VoIP phone.

I have found a discussion about similar problem with Aastra phones: Aastra Internal phones losing registration and showing no service

It seems there could be some firmware problem, however there is no firmware update available for my rather old model of Aastra phone.

So my question is:

  • is there any other solution for this problem of loosing connectivity?
  • and/or: which VoIP phone should I buy instead of Aastra? I want something that surely works with RasPBX, however, I am looking for budget phone though.

Don’t use a VPN. Use a model of desk phone that supports TLS and SRTP if you require encryption.

I am not using VPN because of encryption. Anyway, I have found that there in fact is a problem with ISC-DHCP server as described here.

It seems that connection between RaspberryPi and VoIP problem is lost every couple of minutes.

It seems I have found the solution. I included static IP assignment for eth1 in /etc/dhcpcd.conf file (the DHCP client configuration file) and now VoIP phone is keeping connection for more than two hours. I will see how stable is everything after a day or two, but it seems the problem is solved.